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Author Topic: EEVblog #629 - How To Design a Microphone Preamplifier  (Read 7368 times)

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Online EEVblog

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EEVblog #629 - How To Design a Microphone Preamplifier
« on: June 13, 2014, 04:59:02 PM »
Part 7 of Microphone Technology with Doug Ford, former head designer at Rode Microphones.
This video Doug takes you through the design of the Rode NT3 and NT1000 microphone designs.
There is a ton of stuff in here:
Closed loop unity gain JFET and bipolar follow circuits.
Bootstrapping out miller capacitance.
JFET biasing.
How to eliminate expensive high value resistors.
How to add a bridged-T network to add marketing "pizazz" to a microphone.
How to stop oscillation in closed loop systems.
How to test and design closed loop systems for stability.
Dominate pole compensation.
Traps for young players in power amplifier stability.
How circuits with capacitive loads oscillate.
Emitter degeneration.
Parasitic inductances and their effects.
How to design a low noise and ultra high dynamic range valve based microphone preamplifier.
Microphone power supplies.
And how to design an amplified zener current source.

All other videos in this series are HERE: http://www.youtube.com/playlist?list=PLvOlSehNtuHv98KUcud260yJBRQngBKiw

 

Offline Dave Turner

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #1 on: June 13, 2014, 10:39:17 PM »
This is the latest of an excellent series. More please!
 

Offline dentaku

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #2 on: June 13, 2014, 10:55:55 PM »
Adding pizazz to a mic seems like it's just something recording engineers have gotten used to over the years so they want all mics to have the boost in the higher frequencies.
I'd much rather have a mic that's flat and deal with it somewhere in the chain AFTER the mic. It's not like an instrument such as a synth where the imperfections give it a distinct sound, it's a measuring device :)
 

Offline JackOfVA

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #3 on: June 13, 2014, 10:59:34 PM »
The most interesting of the series so far - Doug both understands circuit design and can why things are done they way they are. That's an uncommon combination.
 

Offline krivx

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #4 on: June 13, 2014, 11:08:32 PM »
Adding pizazz to a mic seems like it's just something recording engineers have gotten used to over the years so they want all mics to have the boost in the higher frequencies.
I'd much rather have a mic that's flat and deal with it somewhere in the chain AFTER the mic. It's not like an instrument such as a synth where the imperfections give it a distinct sound, it's a measuring device :)

I don't know, while a lot of people would agree with you about a flatter response, microphones and preamps are certainly used for distinct sounds. The goal is a recording that meets certain requirements in the sound and accuracy usually isn't one of them. Typically single-point sources at different positions in space will be mixed together, so there's no real analog to how a human hears anyway.  There are reference microphones designed and marketed as "measurement" devices but they don't seem to get nearly as much studio use.
 

Offline mikeselectricstuff

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #5 on: June 13, 2014, 11:31:26 PM »
I was waiting for the graph of excitement vs. pizazz vs. frequency...
What would the units be - the millipssst ?
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Offline mux

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #6 on: June 13, 2014, 11:34:37 PM »
It's funny that Doug should mention how much power the tube amp actually draws. It's really no wonder he's getting such good DR performance out of it.

At that point it's really just all about information power. Information power (=Shannon-Hartley theorem, yes, it is just as relevant to analog signals as it is to digital wireless transfer) is going to do you in with any low-power circuit. 143dB of dynamic range at a 20kHz bandwidth is going to require something in the order of megabits per second of information-equivalent, and depending on how it was actually measured about the equivalent of 3-10 times as much in a purely analog system (because noise spectral density over the audio range is weighted). Even the best opamps don't perform much better than the order of hundreds of nJ/bit up to a few µJ/bit, so you would indeed expect to need in the order of watts to reach this kind of performance.

Quick random calculation: LT1115 gets 143dB dynamic range for an output level of 3.3V and unity gain, with the opamp operating at +/-15V and 10mA, i.e. 0.3W. This is not an ideal opamp to do the same kind of job as Doug's design (it actually can't perform as well over the entire audio range), so assume you're going to need something beefier and more power hungry in the real world. This represents at best 0.3µW/bit performance (assuming 980kbit/s analog information-equivalent). Pretty much one of the best opamps you can get, and including power supply it's going to easily eat through a couple watts of power in a real world application.
 

Offline Jacko

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #7 on: June 14, 2014, 12:04:24 AM »
Something like the Vishay-Siliconix SSTD89A?

http://www.symmetron.ru/datasheet/vishay/active/71092.pdf

regards, Jack

 

Offline SNGLinks

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #8 on: June 14, 2014, 02:19:22 AM »
As a microphone service engineer for Sennheiser's UK agent in the 70s I found this series fascinating!
 

Offline Zad

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #9 on: June 14, 2014, 01:42:21 PM »
I always wondered where supremo music producer Trevor Horn had got to!



Absolutely top stuff guys, don't hesitate to get Doug (or other engineers) back in to talk about in-depth techy stuff. I'll certainly remember that power supply.

Online EEVblog

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #10 on: June 14, 2014, 01:57:50 PM »
This is the latest of an excellent series. More please!

Sorry, there is no more.
Well, maybe one short mic teardown I think, but that's it for the theory.
 

Offline zucca

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #11 on: June 14, 2014, 10:59:47 PM »
In the era of Arduino & raspberry pi this is like someone open a window to let fresh analog air in.
Finally! Please more analog! There is enough www in digital....
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Can't love what you don't know. Zucca
 

Offline parbro

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #12 on: June 16, 2014, 01:52:16 AM »
Question about the power supply he was describing at the end of the video. He showed a power zener circuit. I assume this was for the -120 volt supply. What value zener would that require? Can you get 120V zeners?
 

Offline SeanB

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #13 on: June 16, 2014, 02:06:53 AM »
Yes, you get zeners in almost any arbitrary voltage, though above about 6V they are actually avalanche diodes. 120V is quite common, though if you remember in the video it is a simplified drawing, it actually is a TL431 and a power mosfet that emulates a high power zener diode.
 

Offline markb82

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #14 on: June 16, 2014, 01:08:05 PM »
I'm a bit confused.  Are these JFETs or MOSFETs?  As far as I've learned JFETs don't have isolated gates, while on the white board the FETs are drawn with isolated gates.  Also the zener 'booster' circuit doesn't work unless the FET gate is isolated, i.e. it has to be a MOSFET.
 

Offline T3sl4co1l

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #15 on: June 16, 2014, 02:51:34 PM »
Everyone screws up FET symbols 100% of the time...(except me, of course, obviously!..)...

Typical mistakes:
- JFET with insulated gate (see above)
- MOSFET drawn as a BJT with square leads (including the "emitter" arrow pointing down incorrectly indicating current flow in the switched direction, not the forward-bias direction)
- Same but with insulated gate (it's a start?)
- MOSFET drawn without source or drain specified (not always a mistake; common in monolithic diagrams, where S/D are usually symmetrical; PMOS is indicated with a negation circle on the gate pin)
- Drawing the gate as a straight line, with the pin joining randomly at the top, middle or bottom
- Drawing the channel variously as a straight line or three line segments
- Drawing the parasitic body diode (often as a zener) redundantly (the diode is already indicated, that's the triangle pointing at the center line segment, which is the substrate connection)

The list goes on.

Supposedly, there's a distinction between connecting the gate at the center vs. at the source end (never the drain end), or the channel being a single line (depletion mode: think continuous line = normally-on) or segments (broken line = normally-off), though I don't think that convention is used very often if at all (believe the only place I read it explicitly is an old ARRL Handbook?).  Depletion MOS is uncommon enough that I would suggest specifying it by part number rather than by general characteristics and always use the conventional MOSFET symbol (three line segments, gate connects at the bottom, no redundant reverse diode).

This has been your daily dose of pedantism ::)

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Need engineering assistance? Drop me a message!
 

Offline zucca

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #16 on: June 17, 2014, 02:09:44 AM »
T3sl4co1l

can you put a tutorial together with pictures?

TIA

Can't know what you don't love. St. Augustine
Can't love what you don't know. Zucca
 

Offline gxti

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #17 on: June 17, 2014, 02:35:54 AM »
https://en.wikipedia.org/wiki/MOSFET#Circuit_symbols

Personally I have the "redundant" diode in my symbols because it makes it blindingly obvious to me which way around it needs to go. Making fewer mistakes saves time and money, which is far more important than pleasing the purists.
 

Offline kizzap

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #18 on: June 17, 2014, 03:25:59 PM »
did anyone else notice the joke at the start with the capacitors? "polypropylene, polystyrene, polyputthekettleon..."  :-DD
<MatCat> The thing with aircraft is murphy loves to hang out with them
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Offline SeanB

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Re: EEVblog #629 - How To Design a Microphone Preamplifier
« Reply #19 on: June 18, 2014, 04:01:04 AM »
Great, good, whatever............. performance and DA wise.
 


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